Steady-state diffusion and the cell resting potential

The steady-state diffusion of ions through separate, selective channels is described according to irreversible thermodynamics. Ion fluxes thus obtained are the same as those in the parallel conductance model. The equivalent electric circuit set up to describe the system has its electromotive forces expressed by the chemical potentials of the diffusing ions. The expression obtained for the potential differs from the Goldman-Hodgkin-Katz formula, and is reputed to be more accurate. In order for the passive diffusion flows to remain steady, active transport mechanisms must pump the ions up their electrochemical potentials. Such pumps have been incorporated into the equivalent circuit. They supply energy lost in the dissipation caused by preexisting passive forces without affecting the potential, which can thus hardly be called passive diffusion potential. Ion pumps can also create an electric potential in excess of that by passive forces, especially when secondary active transport is involved. The same equivalent circuit is, however, able to describe the whole range of seemingly different situations - from passive diffusion of an electrolyte to active extrusion of anions from the living cell. It has been applied to explain the measured plasma membrane potential of cells, especially those whose potential does not behave as the potassium electrode.